Temperature-stabilized amplifier circuit

ABSTRACT

A circuit for amplifying an input voltage (VIN) into an output voltage (VOUT) with an overall gain factor, which is a product of a first gain factor (S) and a second gain factor (R 1 ), comprises means for generating an intermediate signal (I 2 ) from the input voltage (VIN) and the first gain factor (S) and means for generating the output voltage (VOUT) from the intermediate signal (I 2 ) and the second gain factor (R 1 ) first gain factor (S) and the second gain factor (R 1 ) have opposite temperature dependencies.

The invention relates to a circuit for amplifying an input voltage intoan output voltage with a temperature-stabilized gain factor.

Amplifier circuits often have the problem that they containtemperature-dependent components, the consequence of which is that theoutput signals of the amplifier circuit are likewise a function of theoperating temperature.

FIG. 1 shows a schematic circuit diagram of an amplifier circuit basedon CMOS (Complementary Metal Oxide Semiconductor) technology. Theamplifier circuit contains a transconductance amplifier OTA, whichconverts an input voltage VIN into a current I2. The current I2 isconverted into an output voltage VOUT by an operational amplifier OPA,which is connected up with a resistor R1 as a current-voltage converterCVC.

In addition to high-impedance inputs 1 and 2, the transconductanceamplifier OTA also has a high-impedance output 3. The transconductanceamplifier OTA therefore behaves like a current source so that thecurrent I2 that can be coupled out at the output 3 is controlled by theinput voltage VIN in accordance with the following equation (1):I 2=S*VIN,  (1)where S denotes the transconductance of the transconductance amplifierOTA and is given by the differential of the current I2 with respect tothe input voltage VIN at the operating point.

Within the transconductance amplifier OTA, the latter has a differentialamplifier stage comprising a current source IREF

and also transistors T1 and T2. The input voltage VIN is present at thegate terminals of the transistors T1 and T2. Furthermore, thetransconductance amplifier OTA comprises three current mirrorsconstructed with transistors T3 and T4, T5 and T6, and T7 and T8. Thetransistors T1, T2, T7 and T8 are n-channel MOSFETs, while thetransistors T3, T4, T5 and T6 are p-channel MOSFETs.

The current I2 is converted into the output voltage VOUT by theoperational amplifier OPA by means of the resistor R1 in accordance withthe following equation (2):VOUT=R 1*I 2+VCM,  (2)where VCM specifies the center voltage. A combination of equations (1)and (2) yields equation (3) as the transfer function of the presentamplifier circuit:VOUT=S*R 1*VIN+VCM,  (3)where the product of the transconductance S and the resistance R1specifies the gain factor of the amplifier circuit.

In CMOS fabrication processes, linear resistors are often produced bydeposition of polysilicon material. The temperature coefficient of suchresistors which specifies the change in resistance with temperature, iscorrelated with the resistance per unit area of the polysilicon. In thecase of only small resistances per unit area, the temperaturecoefficient is positive. The temperature coefficient decreases as theresistance per unit area rises and becomes negative for largeresistances per unit area.

In the case of the present amplifier circuit, the resistor R1 must havea high resistance. Otherwise, the transconductance S of thetransconductance amplifier OTA would have to be large in ordernonetheless to obtain

an acceptable gain factor of the amplifier circuit. However, this wouldin turn entail an unacceptably large current consumption of thetransconductance amplifier OTA.

A high-value resistor R1 is fabricated by means of polysilicon with ahigh resistance per unit area in CMOS technology in order to avoid anexcessively large area consumption. What is disadvantageous about this,however, is the resulting negative temperature coefficient of theresistor R1. Since the transconductance S of the transconductanceamplifier OTA likewise decreases as the temperature rises the gainfactor of the present amplifier circuit is greatlytemperature-dependent. This property of the amplifier circuit isparticularly disadvantageous if the amplifier circuit is operated in awide temperature range. Moreover, fabrication tolerances of thehigh-value resistor R1 also influence the gain factor.

The present amplifier circuit is also utilized to amplify AC voltages.The open-circuit frequency of the amplifier circuit likewise depends onthe gain factor. In the case of a gain factor that is unstable over acertain temperature range, this leads to stability problems in theamplifier circuit.

Previous solutions to the abovementioned problems comprise the use of aresistor R1 with only a small temperature coefficient and lowfabrication tolerances and also the use of a temperature-independentcurrent source IREF, thus resulting in only a low temperature-dependenttransconductance S of the transconductance amplifier OTA. Although thetemperature dependence of the gain factor is minimized by this solutionapproach, the area of the circuit becomes large because it is necessaryto use polysilicon resistors with a low resistivity, or else it isnecessary to produce temperature-stable high-value resistors

by means of additional process steps. Both measures are complicated andcost-intensive.

In a further solution approach, in contrast to the solution approachjust described, the temperature independence of the current source IREFis dispensed with and only a resistor R1 with a small temperaturecoefficient and low fabrication tolerances is used. However, theamplifier circuit is permitted to be operated only in a relatively smalltemperature range in this case.

Therefore, it is an object of the invention to provide an amplifiercircuit which has a temperature-stabilized gain factor over a widetemperature range. In particular, the temperature stability of the gainfactor is intended to be ensured also when using high-value resistorswith large temperature coefficients.

A formulated object on which the invention is based is achieved by meansof the features of the independent patent claims 1 and 9. Advantageousdevelopments and refinements are specified in the subclaims.

By means of the amplifier circuit according the invention, an inputvoltage is converted into an output voltage by means of an overall gainfactor. The overall gain factor is a product of a first gain factor anda second gain factor, the first and the second gain factor havingopposite temperature dependencies. The amplifier circuit comprises firstmeans, which serve for generating an intermediate signal from the inputvoltage and the first gain factor. Furthermore, the amplifier circuitcomprises second means, which generate the output voltage from theintermediate signal and the second gain factor.

The advantage of the amplifier circuit according to the invention isbased on the opposite temperature dependencies of the first and secondgain factors. In the event of a change in temperature, one of the twogain factors rises and the other gain factor falls, thus resultingoverall in an essentially temperature-independent product of the twogain factors and, consequently, a temperature-stabilized overall gainfactor.

Consequently, the amplifier circuit according to the invention achievesa compensation of the temperature dependencies of the two gain factors,thus resulting in a temperature independence of the overall gain factorover a wide temperature range. In the case of the previously knownamplifier circuits described above, the temperature dependence of theoverall gain factor was only minimized, but not compensated for.Furthermore, a temperature-dependent variation of the transfer frequencyis suppressed by the amplifier circuit according to the invention.

If the first and second means, for generating the gain factors, havehigh-value resistors with large temperature coefficients, atemperature-independent overall gain factor nonetheless results onaccount of the compensation according to the invention of thetemperature dependencies of the two gain factors.

The first means advantageously have a transconductance amplifier, towhich the input voltage is fed at its inputs and which outputs theintermediate signal in the form of an intermediate current at itsoutput. Furthermore, the first means preferably comprise a currentsource, which provides the current required for operating thedifferential amplifier contained in the transconductance amplifier.

In accordance with an advantageous refinement of the invention, thesecond means contain a current-voltage converter having an operationalamplifier and a converter resistor connected into the feedback path ofthe operational amplifier. The current-voltage converter serves forconverting the intermediate current generated by the transconductanceamplifier into the output voltage.

A particularly advantageous refinement of the invention is characterizedin that the current provided by the current source and the converterresistor have opposite temperature dependencies. The temperaturedependence of the current provided by the current source is transferredto the temperature dependence of the transconductance of thetransconductance amplifier. In accordance with equation (3) atemperature-stabilized overall gain factor of the amplifier circuitresults from the abovementioned requirement.

For the circuitry realization of a current source whose current has anopposite temperature behavior to that of the converter resistor, it isparticularly advantageous if the current source comprises a currentsource bank to whose input transistor a temperature-stabilized inputcurrent is fed and which has at least two output transistors connectedin parallel. The output currents generated by the drain-source paths ofthe output transistors flow through at least one resistor and jointlyfeed the differential amplifier of the transconductance amplifier.

By virtue of the at least two output transistors being connected inparallel, the gate-source voltage of the at least two output transistorsis lower than the gate-source voltage of the input transistor. Thevoltage difference generated on account of the different gate-sourcevoltages is dropped across the at least one resistor. In this case, thelarger the number of output transistors connected in parallel, the

larger the voltage dropped across the at least one resistor.

The temperature dependence of the current which is generated by thedrain-source paths of the at least two output transistors and feeds thedifferential amplifier of the transconductance amplifier is determinedby the temperature dependence of the at least one resistor, to beprecise the temperature dependence of said current behaves oppositely tothe temperature dependence of the at least one resistor. As a result,the temperature dependence of the intermediate current also behavesoppositely to the at least one resistor and thus likewise oppositely tothe converter resistor, which results in a temperature-stabilizedoverall gain factor.

If the input transistor and the at least two output transistors arecoordinated well with one another, voltage variations caused byfabrication tolerances are thereby eliminated as well.

A temperature-stabilized input current can be generated particularlysimply for example by a BGR (Band Gap Reference) circuit. Such circuitsare already present in many devices and may advantageously also be usedfor the amplifier circuit according to the invention.

The amplifier circuit according to the invention is particularlysuitable for being integrated on a common substrate and being fabricatedin particular by means of CMOS technology. In such a fabricationprocess, both the converter resistor and the at least one resistor ofthe current source can be produced by deposition of polysilicon materialwith a high resistance per unit area. This ensures a low currentconsumption of the transconductance amplifier and a good temperaturestability

of the overall gain factor. Furthermore, in this case, the two linearresistors have comparable (negative) temperature coefficients andfabrication tolerances.

The method according to the invention serves for amplifying an inputvoltage into an output voltage. In this case, the overall gain factor,which specifies the amplification of the input voltage into the outputvoltage, is a product of a first and a second gain factor. The first andthe second gain factor are characterized by opposite temperaturedependencies. In the method according to the invention, firstly anintermediate signal is generated from the input voltage and the firstgain factor. Afterward, the output voltage is generated from theintermediate signal and the second gain factor.

The advantage of the method according to the invention again resides inthe opposite temperature dependencies of the first and the second gainfactor. This results overall in an overall gain factor which isstabilized over a wide operating temperature range.

The invention is explained in more detail below with reference to thedrawings, in which:

FIG. 1 shows a schematic circuit diagram of an amplifier circuit inaccordance with the prior art; and

FIG. 2 shows a schematic circuit diagram of an exemplary embodiment ofthe current source according to the invention for feeding thedifferential amplifier of the transconductance amplifier shown in FIG.1.

FIG. 2 shows a schematic circuit diagram of an exemplary embodiment of acurrent source IREF according to the invention, which serves forproviding a current I1, which is used

for the differential amplifier stage of the transconductance amplifierOTA as shown in FIG. 1. The current source IREF contains a transistorT9, the source-drain path of which is connected between a current sourceICONST and a common fixed potential, in particular a ground VSS. Thetransistor T9 is connected as the input transistor of a current sourcebank to transistors T10 and T11 as output transistors via their gateterminals. Furthermore, the drain terminal of the transistor T9 isconnected to its gate terminal. The transistors T10 and T11 arerespectively connected to one another at their drain and sourceterminals. A resistor R2 is connected between the source terminals ofthe transistors T10 and T11 and the ground VSS. The current I1 isgenerated on the side of the drain terminals of the transistors T10 andT11. The transistors T9, T10 and T11 are n-channel MOSFETs, by way ofexample.

A precondition for the functioning according to the invention of thepresent circuit is that the current source ICONST provides atemperature-independent current. This precondition is met for example bya BGR circuit. The current generated by the current source ICONST ismirrored into the drain-source paths of the transistors T10 and T11 bythe transistor T9.

In this case, the gate-source voltage of the transistor T9 is largerthan the gate-source voltages of the transistors T10 and T11. Thedifference between these gate-source voltages is dropped across theresistor R2. The more transistors are connected in parallel like thetransistors T10 and T11 as output transistors of the current sourcebank, the larger the voltage dropped across the resistor R2. The larger,too, the temperature dependence of the current I1 on the resistor R2.,since the current I1 is a function of 1/R2.

Furthermore, the transconductance of a CMOS transistor is proportionalto the current flowing through its drain-source path

if the CMOS transistor is operated below the threshold voltage. Sincethe optimum operating point for the differential amplifier stage havingthe CMOS transistors T1 and T2 is situated between the subthresholdvoltage range and the voltage range with slight inversion, thetransconductance S of the transconductance amplifier OTA is likewiseproportional to the current I1 and, in accordance with the paragraphabove, is thus a function of 1/R2. It follows from this that the gainfactor of the amplifier circuit as shown in FIG. 1 with the presentcurrent source IREF according to the invention is proportional to theresistor R1 and indirectly proportional to the resistor R2. Since theresistors R1 and R2 have identical temperature dependencies on accountof their design, the gain factor is temperature-independent over a widetemperature range.

1. An amplifier circuit with a temperature-stabilized overall gain factor, which specifies the amplification of an input voltage (VIN) into an output voltage (VOUT) and which is a product of a first gain factor (S) and a second gain factor (R1), the amplifier circuit having: first means for generating an intermediate signal (I2) from the input voltage (VIN) and the first gain factor (S), and second means for generating the output voltage (VOUT) from the intermediate signal (I2) and the second gain factor (R1), and the first gain factor (S) and the second gain factor (R1) having opposite temperature dependencies.
 2. The amplifier circuit as claimed in claim 1, characterized in that the first means have a transconductance amplifier (OTA), at whose inputs (1, 2) the input voltage (VIN) is present and at whose output (3) the intermediate signal can be coupled out in the form of an intermediate current (I2).
 3. The amplifier circuit as claimed in claim 2, characterized in that the first means have a current source (IREF), which feeds the differential amplifier of the transconductance amplifier (OTA).
 4. The amplifier circuit as claimed in claim 2 or 3, characterized in that the second means have an operational amplifier (OPA), which is connected up with a converter resistor (R₁) as a converter (CVC) and converts the intermediate current (I2) into the output voltage (VOUT).
 5. The amplifier circuit as claimed in claims 3 and 4, characterized in that the current (I1) provided by the current source (IREF) and the converter resistor (R1) have opposite temperature dependencies.
 6. The amplifier circuit as claimed in one or more of claims 3 to 5, characterized in that the current source (IREF) comprises a current source bank whose input transistor (T9) is fed with a temperature-stabilized input current (ICONST) and which has at least two transistors (T10, T11) connected in parallel as output transistors, the output currents generated by the drain-source paths of the output transistors (T10, T11) flowing through at least one resistor (R2) and jointly feeding the differential amplifier of the transconductance amplifier (OTA).
 7. The amplifier circuit as claimed in one or more of the preceding claims, characterized in that the first and the second means are integrated on a common substrate, and the amplifier circuit is, in particular, a CMOS amplifier circuit.
 8. The amplifier circuit as claimed in claims 6 and 7, characterized in that the converter resistor (R1) and the at least one resistor (R2) of the current source (IREF) are integrated linear resistors which have, in particular, a high temperature coefficient.
 9. A method for amplifying an input voltage (VIN) into an output voltage (VOUT) with a temperature-stabilized overall gain factor, which specifies the amplification of the input voltage (VIN) into the output voltage (VOUT) and is a product of a first gain factor (S) and a second gain factor (R1), the first gain factor (S) and the second gain factor (R1) having opposite temperature dependencies, and the following steps being implemented: (1) generation of an intermediate signal (I2) from the input voltage (VIN) and the first factor (S); and (2) generation of the output voltage (VOUT) from the intermediate signal (I2) and the second gain factor (R1). 